For aircraft, missiles and other platforms, there is frequently a need to provide communications, navigation, IFF (collectively, "CNI"), and other services through antennas that may be flush-mounted to the surface of the platform for aerodynamic and other reasons. A number of antenna structures have been employed to provide the necessary antenna gain over a broad range of frequency, polarization, and beam shape requirements in a form factor suitable for flush mounting.
Of these antenna structures, a planar spiral antenna has two outwardly-spiraling branches lying in the same plane that are symmetrical with respect to a point at the center of the antenna. To produce maximum radiation in two directions that are mutually symmetrical about the plane of the spirals, the two branches are fed out of phase with each other. Each of the two spirals may be terminated resistively at the outward ends of the spirals.
To maximize radiation in a single direction from a spiral antenna, the spirals may be backed by a coaxial cavity extending to the outer edge of the spirals and having a depth equal to about one-half wavelength at the center operating frequency of the antenna. The back surface of the cavity reflects the radiation directed in the back direction so as to reinforce the radiation in the forward direction over a limited frequency range.
Another common type of antenna is an annular slot antenna which comprises an annular slot cut in a metallic surface. A simple annular slot antenna may be formed by terminating a coaxial line to a ground plane such that the coaxial line is open-circuited at the termination. In other words, the coaxial line's center conductor terminates in the circular conducting disk inside the annular slot and the coaxial line's outer conductor or shield terminates in the ground plane outside the annular slot.
A truncated annular slot antenna may be constructed by dividing the annular slot into slot sections which together approximate a full annular slot. For example, the ground plane outside the annular slot may be extended to meet the circular conducting disk inside the annular slot with symmetrical thin fingers of the ground plane. Feeding each of the truncated slots formed thereby in phase and with equal amplitude will approximate the excitation of a full annular slot by a coaxial line.
Because a range of diverse CNI services must be provided on modern aircraft, missiles, and other platforms, requirements for several frequencies, polarizations, and beam shapes are often presented to the antenna suite designer. Unfortunately, conventional antenna suite designs typically employ individual antennas to meet each antenna requirement and do not structurally integrate the antenna suite in a conformal package to thereby minimize space, cost and weight and reduce aerodynamic drag.
Known attempts to provide multiple antenna functions in a single antenna aperture or to structurally integrate an antenna suite have met with limited success. For example, U.S. Pat. No. 5,160,936 to Braun et al. ("Braun") discloses a lightweight phased array antenna system that is conformable to an aircraft fuselage and that combines air-filled cavity-backed slots with printed circuit elements for operation in two or more frequency bands. In Braun, the printed circuit elements are separated from a conductive ground plane in which the slots are cut by a dielectric honeycomb material. The slots and printed circuit elements are individually excitable by a multiband feed network and transmit/receive modules for operation in the UHF band and S band or L band, respectively.
An attempt at structural integration of several antenna apertures is disclosed in U.S. Pat. No. 5,650,792 to Moore et al. ("Moore"). Moore discloses a monopole VHF antenna and a volute GPS antenna housed in a base shell similar to that of a single VHF whip-type antenna. A further attempt at structural integration of several apertures is disclosed in U.S. Pat. No. 4,329,690 to Parker ("Parker"). Parker discloses a multiple antenna system for a ship mast top with the individual antenna sections being in stacked relationship. The separate GPS, TACAN and JTIDS antennas are isolated by decoupling chokes to permit each antenna to rotate about the mast freely. Additionally, a primary radar antenna integrated with an IFF antenna and particularly suitable as a combined primary radar/IFF antenna for smaller vehicles is disclosed by U.S. Pat. No. 4,329,692 to Brunner.
While a number of antenna designs have been developed in an attempt to provide multiple antenna functions in a structurally integrated antenna suite, none of these antenna designs have provided the combination of functionality and structure demanded by some current applications. For example, none of the conventional designs provide the three or more radiating apertures required for CNI services in a conformal geometry.